V. TEMPERATURE DETERMINATIONS 163 



thermometer, enclosing a thermometer in a blackened sphere, or 

 blackening a bulb filled with water, alcohol, or ether and noting in 

 each case the heights to which the thermometric substance rises when 

 radiation is allowed to impinge on the blackened surface. This type 

 of radiometer is insensitive and slow, but because of ruggedness and 

 simplicity has received considerable attention, as, for example, the 

 Vernon globe thermometer. 



The amount of energy received by a radiometer is often so small 

 that electronic or optical amplification is necessary. This energy de- 

 pends upon the difference in temperature between the radiometer and 

 the surface being measured and upon the emissive power of the sur- 

 face. The net transfer of heat by radiation can be written : 



Hr = SoeMT' - Tt)A (8) 



where jF/r — heat transfer in g. cal./sec. So = Stefan-Boltzmann 

 constant = 1.37 X 10~^^ g. cal./sec. /cm. 2, T and To = absolute 

 temperature of hot object and its environment, ei and €2 = emissivity 

 of surfaces of radiator and environment with maximum values of 

 unity, and A = radiating area of object. 



A perfect black body has the highest emissive power or emissivity 

 and is assigned the value of 1. All other objects have emissivities 

 less than 1. Since the emissive power of an object is equal to the 

 absorbing power of the object, emissivity can be related to the re- 

 flecting power (R) of an object with e being equal to 1 — R. This 

 relationship provides a convenient means for measuring the emissivity 

 of biologically important surfaces as, for example, human skin, leaves, 

 animal fur, clothing, etc. Many objects may act as good radiators 

 for one w^avelength and as poor radiators for others. The human 

 skin is a good example. It is important that the emissivity used in 

 the radiation formula be that for the wavelength range in which the 

 object is radiating. The emissivity for white human skin has a value 

 of 0.99 in the infrared portion of the spectrum in which the skin radi- 

 ates — although its emissivity in the visible spectrum would be as low 

 a,s 0.3. 



2. Calibration of Radiometers 



The energy calibration for a radiometer can be most easily made 

 with a radiation standard of the National Bureau of Standards. For 

 low temperature work in the biological field, the calibration can be ac- 



